A roller pump is conventionally known as one of drivers of constant-flowtransferring of a liquid in a flexible tube.
In this roller pump, free rollers stands at uniform intervals on a periphery portion of a disc to be rotated by a motor, which free rollers are supported by respective pivots. Outside surfaces of the free rollers protrude over the disc, and a supporting member is arranged oppositely to the free rollers. The flexible tube is put between the free rollers and the supporting member, and the disc is rotated so that the liquid in the flexible tube can be constant-flow-transferred according to the rotation speed of the disc.
In the field of medicine for example, the above roller pump is used for the artificial dialyzer for circulating the blood, especially when a relatively large quantity of liquid is constant-flow-transferred. When a relatively small quantity of liquid is constant-flow-transferred as in case of dosing liquid medicine by means of an intravenous injection, a drip tube provided between an injection needle and a liquid medicine bag is generally used.
In this way, the reason for using the roller pump for constant-flow-transferring a relatively large quantity of liquid is to carry out the feedback control necessary for securely carrying out the constant-flow-transferring, not because of the limitation of function of a motor.
More specifically, flow fluctuation could generally occur during the constant-flow-transferring of the liquid. When flow fluctuation degree is the same, the larger the flow rate is, the smaller the flow fluctuation rate is. That is, the degree of losing the transferring constantivity is larger when a relatively small quantity of liquid is transferred.
Accordingly, when the motor is continuously rotated with low speed in order to constant-flow-transfer a relatively small quantity of liquid, the motor rotation must be monitored in a shorter period in executing the feedback control, which requires a device to accurately detect the motor rotation number.
However, an encoder, which is used to detect the motor rotation, has structurally a limitation of upper frequency of pulse to be output according to the motor rotation number. Therefore, when the motor rotation speed is too low, the motor rotation number can not be detected with a required high resolution corresponding to the transferring rate when the relatively small quantity of liquid is constant-flow-transferred.
Based on such a background, in the field of medicine, a drip tube has been conventionally used for the small constant-rate dosing of liquid medicine. And, in case that it is difficult to use the drip tube since the dosing amount is too much to use a liquid medicine bag, the roller pump is operated by intermittently driving the motor.
However, though the above operation of the roller pump could keep the above small transferring in a long span of time, the liquid transferring completely stops in a short span of time since the motor intermittently stops. Some liquids or substances to be transferred, however, could not allow such an intermittent transferring. Or, the above intermittent operation could make an operator misunderstand that failure arose, which requires a means to show the intermittent operation.